In 1982, Richard Feynman proposed that a controllable quantum system could be used to simulate other quantum systems more efficiently than conventional computers. See Feynman, 1982, International Journal of Theoretical Physics 21, pp. 467-488, which is hereby incorporated by reference in its entirety. This controllable quantum system is now commonly referred to as a quantum computer, and effort has been put into developing a general purpose quantum computer that can be used to simulate quantum systems or run specialized quantum algorithms. In particular, solving a model for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. David Deutsch observed that a quantum system could be used to yield a time savings, later shown to be an exponential time savings, in certain computations. If one had a problem, modeled in the form of an equation that represented the Hamiltonian of the quantum system, the behavior of the system could provide information regarding the solutions to the equation. See Deutsch, 1985, Proceedings of the Royal Society of London A 400, pp. 97-117, which is hereby incorporated by reference in its entirety.
One limitation in the quantum computing art is the identification of systems that can support quantum computation. The basis for performing quantum computation is a unit, which is hereinafter termed an information device. Information devices can have many embodiments but must fulfill several requirements. One requirement is that the information device must be reducible to a quantum two-level system, which means that it must be able to have two distinguishable quantum states that can be used for computation. The information devices must also be capable of producing quantum effects like entanglement and superposition, described below. In general, the quantum information stored in an information device can, but does not need to be, coherent. A device with coherency has a quantum state that persists without significant degradation for a long period of time, on the order of microseconds or more. One non-limiting example of an information device is a qubit, also termed a quantum bit. A qubit is analogous to a bit in a classical (digital) computer, and is a type of information device that requires coherence. The loss of coherence is referred to herein as decoherence.
The computing power of a quantum computer increases as its basic building blocks, information devices, are coupled together in a controllable manner such that the quantum state of one information device affects the quantum state of each of the information devices to which it is coupled. This form of coupling is referred to as entanglement. Another limitation in the quantum computing art is the identification of methods that can be used to controllably entangle the states of information devices without introducing a significant source of decoherence.